Ferroelectric memories, which are voltage-driven nonvolatile semiconductor memory devices, have preferable characteristics such as high-speed operation, low power consumption, and nonvolatility of retained information during power shutdown. Ferroelectric memories have already been used in IC cards and portable electronic devices.
Ferroelectric random access memories (FeRAMs), which may be typical examples of ferroelectric memories, include a ferroelectric capacitor having a ferroelectric film held between a pair of electrodes, and store information by inducing polarization in the ferroelectric capacitor in accordance with the voltage applied between the electrodes. The information thus written into the ferroelectric film in the form of polarization is retained even after removal of the applied voltage.
In such a ferroelectric capacitor, the polarity of the spontaneous polarization is also reversed in response to the reversal of the polarity of the applied voltage. Accordingly, the written information may be read by detecting this spontaneous polarization. FeRAMs operate with lower voltage than flash memories, and enable writing information at high speed with low power.
It is desirable to repeat heat treatment in an oxygen atmosphere in the process of manufacturing such FeRAMs in order to recover the characteristics of the ferroelectric film degraded by processing in a non-oxidizing atmosphere. Oxygen deficiencies are easily caused in the ferroelectric film of the ferroelectric capacitor by processing in a non-oxidizing atmosphere, so that characteristics as a ferroelectric film, such as the amount of switching charge and a leak current value, may be degraded. Therefore, conventionally, a metal that is less likely to be oxidized even in an oxygen atmosphere, such as Pt, or a conductive oxide such as IrOx or RuOx is used as the upper electrode of the ferroelectric capacitor.
In recent years, FeRAMs are no exceptions to exacting requirements for microfabrication, so that there are also a demand for miniaturization of the ferroelectric capacitor and a demand for adoption of a multilayer interconnection structure. Further, there is also a demand for FeRAMs that operate with low voltage in view of application to portable information processors.
In order for FeRAMs to be operable with low voltage, the ferroelectric film of the ferroelectric capacitor has a large amount of switching charge Qsw. In the case of using a multilayer interconnection structure, however, there is the problem of the degradation of the characteristics of the already formed ferroelectric capacitor due to a reducing atmosphere or a non-oxidizing atmosphere used in the process of forming the multilayer interconnection structure.
For example, in the case of forming the upper electrode with a Pt film or an Ir film, there is a problem in that hydrogen in the reducing atmosphere used in forming an interlayer insulating film in the multilayer interconnection structure enters the Pt film or Ir film to be activated through the catalysis of the metal, so that the activated hydrogen reduces the ferroelectric film in the ferroelectric capacitor. The reduction of the ferroelectric film substantially degrades the operating characteristics of the ferroelectric capacitor. This problem of the degradation of the characteristics of the ferroelectric film is particularly conspicuous in the case of a miniaturized ferroelectric capacitor having its capacitor insulating film formed of a miniaturized ferroelectric film pattern.
The following are examples of related art of the present invention: Japanese Laid-open Patent Publication No. 2004-273787, Japanese Patent No. 3661850, Japanese Laid-open Patent Publication No. 2006-128274, Japanese Laid-open Patent Publication No. 2000-91270, Japanese Laid-open Patent Publication No. 10-242078, Japanese Laid-open Patent Publication No. 2001-127262, Japanese Laid-open Patent Publication No. 2002-246564, Japanese Laid-open Patent Publication No. 2005-183842, Japanese Laid-open Patent Publication No. 2006-73648, Japanese Laid-open Patent Publication No. 2006-222227, Japanese Laid-open Patent Publication No. 2000-58525, Japanese Laid-open Patent Publication No. 2003-197874, Japanese Laid-open Patent Publication No. 2002-289793, and Japanese Laid-open Patent Publication No. 2003-347517.